1. Field of the Invention
The present invention relates to an image forming apparatus for forming an image with use of laser light, and an image forming method for use in the image forming apparatus.
2. Description of the Related Art
Heretofore, there has been generally used a laser scanner equipped with a rotary polygon mirror in an image forming apparatus such as a copier, a printer, or a fax. In the image forming apparatus, the interval of scanning lines can be decreased by increasing the rotation number of the rotary polygon mirror, which enables to increase the number of scanning lines per unit time, thereby enhancing the resolution in the sub-scanning direction. Also, the resolution in the sub-scanning direction can be enhanced by decreasing the moving speed i.e. the rotating speed of a photosensitive member. Japanese Unexamined Patent Publication No. 09-52387 recites an image forming apparatus equipped with a galvano mirror in place of a rotary polygon mirror. The image forming apparatus performs image formation with an increased scanning speed by reciprocal scanning, namely, by causing the galvano mirror to swing back and forth or reciprocate within a certain angular range at a fixed cycle. The image forming apparatus enables to change the resolution in the sub-scanning direction by changing the moving speed of the galvano mirror or the rotating speed of the photosensitive member.
In the image forming apparatus with the increased rotation number of the rotary polygon mirror, various drawbacks, resulting from the increase of the rotation number of the rotary polygon mirror, may occur such as increase of a noise, shortening of the useful life of a motor, or required use of a costly motor. Also, a clock for outputting image data is required to be increased, which may lead to unduly increase of radiation. When the rotating speed of the photosensitive member is decreased, the printing speed may be decreased. Further, in the image forming apparatus with the increased rotation number of the rotary polygon mirror, and the image forming apparatus recited in the above publication, it is necessary to change the moving speed of the rotary polygon mirror, the galvano mirror, or the photosensitive member in order to change the resolution.